This invention relates to methods of manufacturing semiconductor electronic devices, and more particularly to devices, such as multilevel MOS integrated circuits (ICs), incorporating layers of dielectric material and also incorporating, at more than one level, layers of metal.
The oxide layer of an MOS structure must be of relatively high purity. Mobile ions, for example sodium ions, are known to occur in silicon dioxide layers formed by conventional methods. These ions are detrimental to the performance of MOS devices. In silicon field-effect transistors, for example, mobile ions in the gate oxide layer cause shifts in the operating voltage of the device. One conventional, non-destructive method that has been developed for measuring the mobile-ion concentration in silicon dioxide layers is the Triangular-Voltage-Sweep (TVS) method.
Typically, the TVS method is applied to selected test portions of a wafer to be processed. Each of the test portions is configured as an MOS capacitor, having a silicon substrate, a dielectric layer, typically of silicon dioxide, in contact with the substrate, and a metal contact, typically an aluminum contact, formed in contact with the dielectric layer on the side opposite to the substrate. Significantly, two interfaces are defined by these three layers: an interface between the silicon substrate and the dielectric layer, and a second interface between the dielectric layer and the metal contact.
The TVS method is described, for example, in M. Kuhn and D. J. Silversmith, "Ionic Contamination and Transport of Mobile Ions in MOS Structures," J. Electrochem. Soc. Vol. 118,966(1971) and in N. J. Chou, "Application of Triangular Voltage Sweep Method to Mobile Charge Studies in MOS Structures," J. Electrochem. Soc. Vol. 118,601(1971). Briefly, during testing of an MOS capacitor, the capacitor is maintained at an elevated temperature, typically 100.degree.-400.degree. C., while a triangular voltage sweep is applied across the capacitor. That is, the applied voltage is varied linearly with time from a negative extreme of, for example, -10 V to an equal but opposite positive extreme of, for example, 10 V, and then returned to the initial voltage in the same manner. The sweep rate is relatively slow, typically about 5-100 mV per second. (For present purposes, the applied voltage is negative if the silicon substrate is negative relative to the dielectric layer.) As the applied voltage is varied, the displacement current, that is, the time rate of change of the charge induced at the silicon-dielectric interface by the applied voltage, is continuously monitored. The displacement current has, in addition to an electronic component, a component due to the motion of mobile ionic impurities, for example, sodium ions. A graph, here called a characteristic curve, is readily constructed in which the vertical axis represents the displacement current and the horizontal axis represents the sweep voltage (or sweep time, which is typically proportional to the sweep voltage). In many dielectrics, the ionic component due to alkali-metal ions appears in the characteristic curve as a well-defined peak appearing near mid-sweep, that is, near zero applied volts. The area under the peak is proportional to the concentration of mobile ionic impurities.
Additionally, if defects or a conduction path are present in the dielectric, a leakage current may appear. The presence of a leakage current causes the characteristic curve to have a non-zero slope. However, this effect is readily prevented by growing a blocking layer of a highly insulating dielectric, such as high-quality thermally grown silicon dioxide, between the silicon substrate and the dielectric layer to be tested.
In addition to ionic impurities such as sodium, another undesirable impurity in MOS structures is water. Water is an undesirable contaminant in MOS integrated circuits because it can engender protons that may degrade the threshold voltage and transconductance of MOS transistors. In addition, contamination by water is undesirable whenever an aluminum contact is to be deposited in a window etched through the dielectric. Water that is outgassed by the dielectric while the aluminum is being deposited can oxidize the aluminum at the point of contact, thus raising the contact resistance to an unacceptable level.
Contamination by water is a particularly significant problem in low-temperature dielectrics such as spin-on silicon dioxides, which are of interest for forming intermediate dielectrics in vertically integrated VLSI structures. Low-temperature dielectrics are desirable because, for example, they can be deposited at temperatures low enough to avoid diffusion of the aluminum of a first-level metallization layer and formation of aluminum filaments, which could penetrate a shallow p-n junction. However, these materials have relatively low densities (compared, for example, to thermal oxide), and they readily absorb atmospheric moisture.
Thus, a method is needed for fabricating semiconductor devices that includes the step of detecting water in dielectric layers in order to determine whether the water content is within acceptable limits. Until now, however, there has been no non-destructive, wafer-level technique for detecting water in dielectric layers.